Does a tire and wheel weigh substantially more when fully inflated than when it's flat?

thelabdude

06-23-2011, 10:55 PM

No. PV = nRT. An inflated tire will have about 3 times as many air molecules as a flat one. Thus the air molecules will weigh 3 times as much, but 3 times a small number is still quite small. Note, filing with nitrogen in place of mostly nitrogen air makes very little difference in the weight.

FoundWaldo

06-23-2011, 11:38 PM

I guess helium is out, then.

thelabdude

06-24-2011, 09:31 AM

Helium would make a very slight reduction in the unsprung mass, but be prone to leaking out.

Sicks Ate

06-24-2011, 12:44 PM

Related anecdote:

I was doing some remodeling work with a guy who apparently wasn't as sharp as the nails he was driving.

I commented on how heavy his cheap-o pancake config. air compressor was.

His response? "Well sure, it's got 90 lbs. of air in it."

He was serious.

notquitekarpov

06-24-2011, 02:53 PM

:D Fabulous!

Hampshire

06-24-2011, 02:56 PM

Take a deeeep breath and hold it and step on a scale to weigh yourself.
Now exhale completely and weigh yourself again.
Any difference?
I didn't think so.

Malacandra

06-24-2011, 03:23 PM

Take a deeeep breath and hold it and step on a scale to weigh yourself.
Now exhale completely and weigh yourself again.
Any difference?
I didn't think so.

Of course not. The air in you is at atmospheric pressure to a good order of accuracy, so your mass increase on inhalation is accompanied by a volume increase - you displace as much additional air as you breathed in, so buoyancy offsets the increased mass. You would need to weigh yourself in vacuo. :D

Napier

06-24-2011, 03:39 PM

Related anecdote:

I was doing some remodeling work with a guy who apparently wasn't as sharp as the nails he was driving.

I commented on how heavy his cheap-o pancake config. air compressor was.

His response? "Well sure, it's got 90 lbs. of air in it."

He was serious.

I think his comment was pretty reasonable. You could quantify how much compressed air was in a tank by the number of pounds. Air surrounds everything we do, and it isn't that obvious what it weighs, compared with water. We see bottles full of water and empty bottles and have a clear idea what it weighs as a result. But since we basically never handle sizable objects that are void of air, we don't get this chance.

He's wrong of course, but it's not his fault people often incorrectly refer to pressure in "pounds" instead of "pounds per square inch". I can see how somebody who isn't very quantitative or hasn't thought about it much could get where he got.

And, in some other thread somewhere, when I said that air had a density of about 1 in SI units, I think some other posters scoffed and said I had it confused with water. But air IS typically just a tad over unit density in SI, depending on conditions. Apparently it's not so hard to get this muddled.

Chronos

06-24-2011, 03:47 PM

Yet another reason why we ought to ditch MKS and CGS and instead go with the MTS system: Meters, tons, seconds. That way, none of the base units have prefices on them, and the density of water is 1.

Napier

06-24-2011, 04:35 PM

Chronos, if only people would adopt my system, we'd fix these things and more:

A new mass unit named the Einstein would be equal in size to the present kilogram. (Yeah, there's some unit already named the Einstein now, but does anybody actually care about that one? Come on.)

The prefix "kilo" would have "K" as its symbol, rather than "k", so that all larger-than-unity prefixes are uppercase and all smaller-than-unity prefixes are lowercase.

The Greek lowercase "mu" would be replaced by an English "u". That way, all the prefixes are ordinary Western keyboard letters. We should also consider messing with the uppercase Greek "omega" and a few other plump little pigeons.

Hampshire

06-24-2011, 05:04 PM

I wonder if people who think something weighs more when pressurized are thinking along the lines of something like propane tanks for grills not realizing that they have actual liquid in them that gives them weight.

thelabdude

06-24-2011, 09:34 PM

Oh, but cramming more air in a tire does add weight, just very little. At standard temperature and pressure, 22.4 liters of air weighs about 29 grams, I think about an ounce. I am not sure of the volume of a tire, but I don't think at common 30 psi, a regular car tire is going to take that much air to inflate it. More weight, yes. Much, no way.